A wrap spring clutch has a spiral spring surrounding or overlying two arbors, which are cylinders. One of the arbors continuously rotates and drives the other arbor when the spiral spring is held in engagement with the driving arbor. With the driving arbor continuously rotating, the wrap spring clutch is capable of starting and stopping the driven arbor intermittently. Two examples of wrap spring clutches are disclosed in U.S. Pat. No. 3,529,703 to Kroeker and U.S. Pat. No. 4,760,903 to Stegelmeier et al.
When the end of the spring wrapped around the driving arbor is held, the spring is forced to unwind and loosen its grip on the driving arbor. When this occurs, driving of the driven arbor ceases.
When the end of the spring wrapped around the driving arbor is released so as to be able to rotate with the driving arbor, the driving arbor rotates the spring to cause the driven arbor to rotate. The spring has its other end secured to the driven arbor so that it is wrapped down on the driven arbor at all times.
A slight variation in the diameter of the spring or a slight variation in the outer diameter of either or both of the two arbors can occur during manufacturing. To negate the effects of these manufacturing variations, wrap spring clutches have previously been manually adjusted. This adjustment has the potential of misadjustment at a later time to require repairs on the assembly line or in a customer's office.
Furthermore, wear of the wrap spring and the two mating arbors during use can require further adjustment to compensate for this wear. This is because wear increases the time for the clutch to engage.